1. Field of the Invention
The present invention relates to a punch which is employed when forming a two-stage hole configuration, comprising a first stage hole of a given diameter and a second stage hole of a smaller diameter than the diameter of the first stage hole, by forging.
2. Description of the Prior Art
A hose fitting has been formed by cold forging recently. For instance, such a technology is disclosed in Japanese Examined Patent Publication (KOKOKU) No. 45011/1987 and so on.
When forming a hose fitting, a workpiece having a hole portion 100 as illustrated in FIG. 4 is prepared first. Then, a first stage hole is formed, and a second stage hole is formed by using a predetermined punch at the same time. The workpiece is thus made into a configuration as illustrated in FIG. 5. In FIG. 5, a first hole 101 being the first stage hole and having a larger diameter is used as an engagement portion to be engaged with the other mating part after a female thread and the like is formed thereon, and a second hole 102 being the second stage hole, having a smaller diameter and disposed at the bottom portion of the first hole 101 is used as a joint portion to be joined with a nipple, for example, after a through hole is made at the bottom portion.
When processing such a two-stage hole configuration by forging, a punch 200 as illustrated in FIG. 6 has been employed, for instance. The punch 200 comprises a larger diameter portion 201 having the outside diameter corresponding to the diameter of the first hole 101 having a larger diameter, and a smaller diameter portion 202 having the outside diameter corresponding to the diameter of the second hole 102 having a smaller diameter. However, when the end surface of the larger diameter portion 201 is made into a flat surface perpendicular to the axial direction of the punch 200, an oil supplied during the forging is enclosed between the end surface of the larger diameter portion 201 and the workpiece. The enclosed oil makes it hard to precisely carry out the forging.
Accordingly, a punch 300 as illustrated in FIG. 7 has been employed in the forging operation so far. The punch 300 also comprises a larger diameter portion 301 and a smaller diameter portion 302, but the end surface of the larger diameter portion 301 is made into an inclined tapered surface 303. Further, the larger diameter portion 301 has a chamfered portion 304 disposed on the cylindrical surface thereof and extending in the axial direction thereof. An oil remaining between the punch 300 and a workpiece is expelled from the tapered surface 303 to the outside of the workpiece by way of the chamfered portion 304.
However, when processing a such hole by forging employing the above-mentioned punch 300 as illustrated in FIG. 7, the hole portion thus formed comes to have a configuration as illustrated in FIG. 8. Namely, the bottom surface of the first hole 101 cannot be made into a flat surface perpendicular to the axial direction of the workpiece, it is rather made into an inclined tapered surface 103. Further, there is a marking problem. That is, traces 104 resulting from the chamfered portion 304 of the larger diameter portion 301 of the punch 300 are marked on the inner periphery surface of the first hole 101. Consequently, an additional processing such as machining and the like has been further required even after the hole processing has been completed by forging.